A transmission device that generates a modulated signal from transmission data and outputs the modulated signal via a radio antenna is widely used in various implementations. In order to improve an efficiency of an amplifier in the transmission device, a configuration that generates a modulated signal by using pulse width modulation (PWM) is proposed.
The transmission device includes, for example, a square wave modulator 1, an amplifier 2 and a bandpass filter (BPF) 3, as illustrated in FIG. 1. The square wave modulator 1 generates a PWM (pulse width modulation) signal corresponding to an amplitude and a phase of an input modulated signal. A width of a pulse corresponds to an amplitude Ain of the input modulated signal. A timing of a pulse (that is, a position of a pulse in the time domain) corresponds to a phase φin of the input modulated signal. A repetition frequency of a pulse train corresponds to a carrier frequency of an output signal of the transmission device. The amplifier 2 amplifies the PWM signal. Since the PWM signal is a two-level signal (in monopolar PWM), the amplifier 2 can amplify the PWM signal by switching operation. Thus, the amplifier 2 may be implemented by, for example, an efficient class-D high-power amplifier. The BPF 3 extracts a carrier frequency component. According to the configuration, the transmission device can amplify the input modulated signal and transmit the amplified signal. It is preferable that a phase φout of the output signal of the transmission device match the phase φin of the input modulated signal.
As described above, according to a configuration in which an input data signal is converted into a PWM signal on the input side of an amplifier and a bandpass filter is implemented on the output side of the amplifier, an efficiency of the amplifier improves. Note that technologies of processing a signal using PWM are described, for example, in the Documents 1-3 listed below.    Document 1: F. H. Raab, Radio Frequency Pulsewidth Modulation, IEEE Trans on Communications, vol. 21, No. 8, pp. 958-966, August 1973    Document 2: Michael Nielsen et al., An RF Pulse Width Modulator for Switch-Mode Power Amplification of Varying Envelope Signals, Topical Meeting on Silicon Monolithic Integrated Circuit in RF Systems, pp. 277-280, 2007 IEEE    Document 3: Michael Nielsen et al., A 2-GHz GaAs HBT RF Pulsewidth Modulator, IEEE Trans on Microwave technology and techniques, vol. 56, No. 2, pp. 300-304, February 2008
The square wave modulator 1 for generating a PWM signal in FIG. 1 includes a comparator that compares an amplitude information signal to indicate an amplitude of a transmission symbol and a sine wave signal of a carrier frequency. When the sine wave signal is higher than the amplitude information signal, a pulse is output from the comparator. Thus, a pulse width of the PWM signal depends on an amplitude of the transmission symbol.
However, in a case where the transmission device transmits an RF (radio frequency) signal, an operation speed of a conventional comparator is not sufficient. That is, it is difficult for the conventional comparator to generate a pulse of a sufficiently narrow width. Accordingly, when the transmission device transmits an RF signal, the PWM signal generated by the comparator may be distorted.